In the continuing efforts to reduce the carbon emissions of vehicles the steel industry, together with the car manufacturers, continue to strive to steels which allow weight reduction without affecting the processability of the steels and the safety of the finished product. To meet future CO2-emission requirements, the fuel consumption of automobiles has to be reduced. One way towards this reduction is to lower the weight of the car body. A steel with a low density and high strength can contribute to this. At the same thickness, the use of a low density steel reduces the weight of car components. A problem with known high strength steels is that their high strength compromises the formability of the material during forming of the sheet into a car component.
Ordinary high strength steels, for example dual phase steels, allow use of thinner sheets and therefore weight reduction. However, a thinner part will have a negative effect on other properties such as stiffness, crash—and dent resistance. These negative effects can only be solved by increasing the steel thickness, thus negating the effect of the downgauging, or by changing the geometry of the component which is also undesirable.
In U.S. Pat. No. 6,383,662B1 US2010/0300585 low density steel is proposed based on addition of large amount of the light element of aluminium between 6 and 10%. However, the addition of the large quantity of Al has a negative impact on the elasticity modulus (E-modulus). To meet the stiffness requirements of auto body structures, a low elastic modulus of steel has to be compensated by increasing the gauge of the steel. This increases the weight of the part and thereby the weight reduction potential of this type of steel. A known way to increase the modulus of elasticity and reduce the density of steel is by incorporating ceramic particles of different natures, such as carbides, nitrides, oxides or borides. These particles have a much higher elastic modulus, ranging from about 300 to 550 GPa, than that of the steel base which has an E-modulus of around 205-210 GPa.
Powder metallurgy is normally used to introduce these ceramic particles uniformly distributed in a matrix of steel. Despite providing improved mechanical properties in comparison with conventional steels containing no dispersion of ceramic particles, powder metallurgy has severe practical and financial restrictions.
Reactions of the metal powders are difficult to prevent because of the high surface area of the metal powders. Even after compacting and sintering, there may be residual porosity that may play a role in inducing fracture during cyclic loading. Uniform distribution of the particles in the matrix is difficult to achieve. Moreover the chemical composition of interfaces matrix/particle, and therefore their cohesion is difficult to control because of the surface contamination of the powders before sintering. In addition, the cost of the process like power metallurgy is very high. Powder metallurgy process is therefore not economic for production on the scale required for the automotive and construction industry.